It is well known that nickel based superalloys are extensively employed in high performance environments. Such alloys have been used extensively in jet engines and in gas turbines where they must retain high strength another desirable physical properties at elevated temperature of a 100.degree. F. or more.
The following U.S. Patents disclose various nickel-base alloy compositions: U.S. Pat. No. 2,570,193, 2,621,122, 3,046,108, 3,061,426, 3,151,881, 3,166,412, 3,322,534, 3,343,950, 3,575,734, 3,576,681, 4,207,098, and 4,336,312. Despite the number of patents concerning nickel-based alloys, it is still not possible for workers in the art to predict with any degree of accuracy the physical and mechanical properties that will be displayed by these alloys when they are processed using heat treatments and forging practices different from those previously employed.
In addition, nickel-base alloys have been found to be difficult to fabricate, particularly into large forged parts, and consistently obtain fine grain size which aids in achieving mechanical properties in the forged parts.
U.S. Pat. No. 4,793,868 discloses a method for reducing fatigue crack growth in nickel base superalloys. The method involves the step of forming a part to near net shape by forging of by other forming technique. The part is then heat treated to develop regular grains by recrystallization. The part is then deformed by at least 15% to achieve a desired net shape.
Copending U.S. Ser. No. 07/750,127 discloses a process for fabricating recrystallized forged parts from nickel based alloys. The process includes heating the part to a temperature above the recrystallization temperature of the alloy and subjecting the part to a first upset forging operation to provide a reduction in the original length of the body followed by redrawing. The forged body is then subjected to a recrystallization treatment and upset forged a second time wherein its length is reduced at least 25%. Afterwards, the forged body is subjected to another recrystallization treatment and a third upset forging step which provides a reduction in length of at least 75% followed by recrystallization and solution heat treating, quenching and aging. The resulting forged parts having an ASTM grain size of 3 or finer as measured by ASTM E112 Intercept method and a yield strength in the range of 135 to 175 KSI.
Accordingly, it would be advantageous to provide an economical and effective process for achieving fine grain size and improved mechanical properties in large forged parts fabricated from superalloys such as nickel alloys, iron-nickel, nickel-cobalt and cobalt-base alloys and the like without the need for multiple recrystallization treatments.